Operational Air Pollution Modelling

by Tiziano Tirabassi

Air quality management and protection require a knowledge of the
state of the environment encompassing both cognitive and interpretative
elements. The monitoring network, together with the inventory of emission
sources, is of fundamental importance for constructing the cognitive framework
but neglects the interpretative one. Efficient air pollution management
must dispose of interpretative tools which are able to extrapolate in space
and time the values measured where analysers are located. However, the
improvements to the atmosphere can only be obtained using plans that reduce
emissions ie by means of instruments (like air pollution models) able to
link the causes (source emissions) of pollution to the related effects
(air pollution concentrations). We describe a set of air pollution models
developed at the Istituto per lo Studio dei Fenomeni Fisici e Chimici della
Bassa e Alta Atmosfera (FISBAT-CNR).

In practice most of the estimates of dispersion of air pollution from
continuous point sources are based on the Gaussian approach. A basic assumption
for the application of this approach is that the plume is dispersed by
homogeneous turbulence. However, due to the presence of the ground, turbulence
is usually not homogeneous in the vertical direction.

Various organisations world-wide are currently introducing new advanced
modelling techniques based on the results of recent research on the meteorological
state of the boundary layer. These advanced modelling techniques contain
algorithms for calculating the main factors that determine air pollution
diffusion in terms of the fundamental parameters of the atmospheric boundary
layer, such as the Monin-Obukhov length scale and friction velocity. Experimental
work and modelling efforts have attempted to parameterize the surface fluxes
of momentum, heat and moisture in terms of routinely measured meteorological
parameters.

Model Codes

Within this framework, several model codes employing a non-Gaussian
analytical solution of the advection-diffusion equation have been developed
at FISBAT-CNR, Bologna:

KAPPAG ­ This model uses an analytical solution based on the vertical
profiles of wind and diffusion coefficients that are power functions of
height. The model can handle multiple sources and multiple receptors, simulating
time-varying conditions in which each time interval (eg, 1 hour) is treated
as a stationary case. The model output is a statistical summary of the
concentrations computed at each receptor, during each time step, and due
to each source. Partial and total concentrations are computed for hourly
and multi-hour averages. Highest and second-highest values are also evaluated.

KAPPAG-LT ­ This is the climatological version of KAPPAG, insofar
as it produces seasonal and/or annual mean concentrations.

CISP ­ This screen model provides a method for estimating maximum
ground level concentrations from a single point source as a function of
turbulence intensity and wind speed. It is designed for the low-cost, detailed
screening of point sources in order to determine maximum one-hour concentrations
and is regarded as a useful tool for a screen analysis in that it is a
relatively simple estimation technique, providing a conservative estimate
of the air quality impact.

VIM ­ This is a screening model for estimating maximum ground-level
concentrations as a function of turbulence intensity, wind speed and wind
direction, in an area with many emission sources. It is considered to be
a useful tool for screen analysis as it constitutes a relatively simple
evaluation technique that provides a conservative estimate of the air quality
impact of a specific multi-source area and a model for the evaluation of
the maximum ground concentrations produced by many emission sources.

MAOC ­ This is a model for the evaluation of pollutant concentrations
in complex orography. The simulation of terrain-induced distortion of flow
streamlines is accounted for by modifying the effective plume height.

VHDM ­ This practical model evaluates ground level concentrations
from elevated sources, utilising a Fickian-type formula where the source
height is a simple function of the wind velocity and eddy diffusivity vertical
profiles. The model accepts experimental profiles of the above parameters,
as well as the theoretical profiles proposed in the scientific literature,
such as the vertical profiles of the wind and eddy diffusion coefficients
predicted by similarity theory. In the latter case, the model can be applied
routinely using as input simple ground level meteorological data acquired
by an automatic network.

M4PUFF ­ A puff model where the pollutant concentration in puffs
is described through the first four moments of its spatial distribution.
The model is based on a general technique for solving the K-equation using
the truncated Gram-Charlier expansion of the concentration field and the
finite set of equations for the corresponding moments. Currently, the type
A Gram-Charlier expansion is a classical method for approximating a given
distribution with moments of any order, basically consisting of a truncated
expansion in terms of Hermite functions, whose coefficients are chosen
so as to reproduce the sequence of moments of the distribution up to a
given order.

SPM ­ This is a practical model based on similarity theory for
the dispersion of skewed puffs. It utilises approximate solutions proposed
for the dispersion of a cloud of passive contaminant released from an instantaneous
source near the ground and particular importance is accorded to describing
the interaction between wind shear and vertical diffusion and the process
that transforms shear produced skewness into diffusive variance in the
wind direction.

The SPM model has been used in a project between Italy and Brazil to
investigate the dispersion of radionuclides from the Brazilian Navy's industrial
installation 'Centro Experimental ARAMAR' (CEA) located in Iperó,
in a rural region of the State of São Paulo.

The performance of the models has been assessed with success in cases
of both ground level sources, using data of the Prairie Grass experiment
, as well as elevated sources, using data from the EPRI experiment at Kincaid
power plant and from the Copenhagen data set. In addition, performance
in the case of complex terrain has been evaluated using wind tunnel data
from the Fluid Modelling Facility of the US Environmental Protection Agency,
Triangle Park (North Carolina).